SAO PAULO -- Elisangela Santos doesn't understand why everyone in her neighbourhood on the outskirts of Sao Paulo is being told to get vaccinated for yellow fever this year. Yellow fever has long been endemic in parts of Brazil, and she smells a rat.

"Every year, it's something else," the 44-year-old school custodian said as she waiting recently outside a health post in the Jardim Miriam district. "They invent another thing to make Brazilians spend money."

The vaccine is free at public health posts around the country, but Santos' suspicion that someone must be profiting somewhere is typical of the current high levels of mistrust Brazilians hold for officialdom. Flagging faith in Brazil's institutions amid a series of corruption scandals, a chaotic communications campaign promoting the vaccine, and the country's decision to give partial doses to stretch supplies are contributing to rumours that the vaccine is a scam, weak or even dangerous.

That misinformation is scaring people away from the campaign that is trying to vaccinate more than 23 million people in areas of Rio de Janeiro, Sao Paulo and Bahia states that until recently were not considered at risk for yellow fever. Nearly six weeks into the campaign, the Health Ministry says 76 per cent of the target population has been vaccinated -- far off its goal of 95 per cent.

The current atmosphere of uncertainty and rumours around the vaccine is likely to make the last 20 per cent very difficult to reach, hampering the efforts of Latin America's biggest nation to contain its largest yellow fever outbreak in more than three decades.

The reticence could even lead to a sustained outbreak in Brazil's megacities. Brazil hasn't had an urban outbreak since 1942.

"If we have a higher number of infected people with yellow fever and the Aedes aegypti mosquito starts to become infected and transmit yellow fever, it could become urban," said Luiz Antonio Teixeira Jr., Rio de Janeiro state's health secretary. "Everything we're doing is to ensure we don't have urban yellow fever."

Yellow fever has long been endemic in large swaths of Brazil, but it has been advancing in recent years and this is the second outbreak in two years in places where vaccinations for the disease were not routine. During the 2016-2017 outbreak, more than 770 people were infected after nearly a decade during which Brazil saw fewer than 10 cases each year. In the current outbreak, there have already been 846 cases confirmed, of which 260 have died. The outbreak is stressing the health system just a few years after a major outbreak of Zika, which was linked to severe birth defects in babies born to infected mothers.

While rumours have sometimes swirled around past vaccination campaigns, the rise of the WhatsApp messaging service is amplifying misinformation like never before, said Igor Sacramento, a researcher at Fiocruz, Brazil's premier public research institute.

WhatsApp "is a fundamental characteristic of the way we circulate information, news, etc.," said Sacramento, who works in the institute's health communications lab. Since WhatsApp messages come from people known by the recipients, they place a high value on the information shared there, and that means people often accept it without checking it, Sacramento said.

One rumour making the rounds on WhastApp, for instance, says a mutation in the yellow fever virus has rendered the vaccine ineffective, even citing a study published by Fiocruz. That is not true, and the institute put out a statement saying that mutations in the disease have no impact on the vaccine's effectiveness.

Misinformation has even been circulating among health professionals, some of whom have resisted giving partial-dose vaccines, said Ana Goretti, the interim co-ordinator of the immunization program at the Health Ministry. In response, the ministry and all of Brazil's medical associations are preparing a statement that reiterates the safety and effectiveness of the partial dose.

The vaccination efforts have also been chaotic. Earlier this year, Brazilians encountered hours-long waits at some health posts before turnout fell precipitously. Many people outside a health post in Jardim Miriam complained that the first post they went to was out of the vaccine. Another woman said her son got vaccinated without presenting a vaccination card but her daughter was turned away for not having it.

Overcoming concerns about the vaccine is particularly important because yellow fever does not appear to be shrinking back to its usual habitat in Brazil's wilder interior. The Health Ministry is even considering extending routine vaccination to all Brazilians, and it has already decided to offer vaccination to all children starting later this year, Goretti said.

The ministry and state health departments are using Twitter and Facebook to promote the vaccination campaign and to share links to information about the vaccine and the disease. Members of the ministry's social media team have also responded to some concerns on the ministry's Facebook page. But they have not been systematically using social media networks or WhatsApp to combat the rumours circulating there. Instead, officials say they are primarily relying on interviews with the local press to explain that the vaccine is safe, effective and necessary. Health workers in Sao Paulo and Rio have also gone door to door to encourage people to get vaccinated.

The rumours have scared off Manoel da Silva's family. The 57-year-old retiree said his adult children and his wife are refusing to get vaccinated, citing stories of people who were sickened by the vaccine and concerns about the partial doses.

"There are lots of things on the internet," said da Silva, who lives on Sao Paulo's southern outskirts. "They think it's a fraud because it's fractional."

Scientists have expressed confidence that the one-fifth dose works and the World Health Organization has said it can be used in emergencies. It's still unclear how long the protection lasts. Brazilian officials say they have data -- which will soon be published -- that shows the vaccine is good for at least eight years, but others have been more cautious, and more studies on its long-term effectiveness are expected.

The yellow fever vaccine, like all vaccines that use live virus, can cause adverse reactions or even make you sick with a disease similar to yellow fever. Susan McLellan, who is a professor of infectious diseases at the University of Texas Medical Branch, says the yellow fever vaccine may even cause more vaccine-linked disease than other live virus vaccines.

But "in an epidemic setting, in a high risk setting, you're a lot safer with the vaccine than the disease," said McLellan, who until recently was the director of Tulane University's Travel Clinic.

Da Silva, whose family is resisting getting vaccinated, said he, too, had concerns but opted to get it in the end.

"I heard about people getting sick" from the vaccine, he said of his initial reticence. "But if a mosquito bites me, I'm already at risk."

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Associated Press producer Yesica Fisch in Rio de Janeiro contributed to this report.